The design of many of the different vehicle systems used in electric and hybrid vehicles involves a number of considerations that are unique to this class of automobiles. This is particularly true of the energy management systems that control delivery and regeneration of the power used to operate the vehicle's electric motor. This is also true to a lesser extent for the vehicle climate control systems since they do not have the large quantities of available heat that are produced by an internal combustion engine. Moreover, electric and hybrid electric vehicles (which are collectively referred to hereinafter as electric vehicles) typically have at least some features that have no counterpart in automobiles based on the internal combustion engine. For example, to help maximize fuel economy and minimize brake wear, electric vehicles sometimes use regenerative braking to recover kinetic energy back into the vehicle's high voltage battery. The implementation of these various vehicle systems require new approaches that seek to minimize the waste of available energy, regardless of whether that energy is in the form of heat or electrical or mechanical energy.
One of the problems encountered in the design of electric vehicle climate control systems is that the vehicle motor and other components typically do not generate enough heat to meet the requirements of interior cabin heating. This is often true for both purely electric vehicles as well as hybrid electric vehicles which may have additional heat available from an auxiliary power unit (APU) that commonly takes the form of a gas turbine or other heat engine. To provide the additional heat, some electric vehicles have utilized electric immersion heaters placed in the coolant loop. However, the use of these electric heaters are typically disfavored, as they are usually designed to have a capacity as high as about 5 kW and, consequently, have an adverse effect on the fuel economy and operating range of the vehicle. Instead, fuel fired heaters are sometimes used which, while more expensive, provide better fuel economy.
Many of the other design considerations important in an electric vehicle relate to use of the high voltage battery that provides the operating power needed by the vehicle's electric motor. Typically, a three-phase motor is used and is connected to provide drive power to two of the vehicle wheels. The motor is run using an inverter that is connected to receive 360 volt dc operating power from the high voltage battery by way of a high voltage supply bus. This high voltage bus is also used to provide operating power for other vehicle systems, including electronic power steering, the climate control compressor motor, and the traction control system. The electronics for these systems are typically located in a power electronics bay (PEB) using a high voltage cable harness. To protect against shock during servicing of the vehicle, a control line is often daisy-chained through this harness in a series fashion so that, upon disconnection of any of the connectors carrying the high voltage bus, the series connection will be broken and a double pole relay is then used to break the battery's positive and negative terminal connections from the high voltage bus. One problem with this arrangement is that it does not account for the large energy storage capacitors that are typically connected across the high voltage bus to control the ripple current on the bus. These capacitors can store a significant amount of energy and therefore pose a risk of shock even after battery disconnection. To discharge these capacitors, a discharge resistance is sometimes connected to the high voltage bus through a transistor switch which is closed as soon as the bus is disconnected from the battery.
Another problem that exists with the use of the high voltage bus is that it sometimes experiences high voltage spikes when inductive components are disconnected or switched off. These higher voltages can be damaging to both the high voltage battery and the electronic components used in the PEB. Moreover, for hybrid vehicles using a heat engine APU, the power generated by the APU cannot be unloaded quickly due to the thermal time constant of the system. Thus, the APU sometimes generates excess electric energy beyond that which can be used in operating the electric motor or charging the vehicle battery. This can result in an overvoltage condition on the high voltage bus, again potentially damaging the battery or other electrical components.
As mentioned above, electric vehicles sometimes utilize a regenerative braking system in which the electric motor is used as a generator to return some of the vehicle's kinetic energy back to the high voltage battery. In this situation, rather than the battery being used to supply operating power to run the electric motor, it is recharged and thus acts as a load on the motor, slowing the vehicle with or without the assistance of the manual brakes. This regenerative braking is used whenever possible, as it returns energy to the high voltage battery and helps prolong the life of the manual brakes. In some instances, regenerative braking cannot function because, for example, the battery is already fully charged or the vehicle is at too low a speed for the motor to generate a high enough voltage to recharge the battery. In these instances manual braking is used even though the regenerative braking would otherwise be capable of providing the necessary amount of speed reduction.
It is a general object of this invention to improve the energy management system of electric vehicles. Preferably, it is also an object of this invention to provide auxiliary cabin heating in a manner that captures available and otherwise wasted electric energy from other systems of the vehicle. It is also preferably an object of the invention to provide protection of the high voltage bus used in electric vehicles and to reduce the risk of shock from the bus when a connector or the battery is disconnected. It is also preferably an object of this invention to provide electric braking of the vehicle in instances when regenerative braking is undesirable and manual braking is unnecessary.